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Advanced Topic 47 of 52 Data Structures

Cuckoo Hashing

Hashing scheme with two tables guaranteeing constant lookup.

How it works

In cuckoo hashing each key can live in one of two positions given by two hash functions, often in two tables. Lookup checks at most two slots, giving worst-case O(1) time. Insertion places the key in one slot, evicting any occupant to its alternative slot, possibly cascading; if a long cycle is detected, the table is rehashed with new hash functions.

Where it is used

Network hardware lookup tables, high-performance caches, in-memory databases and packet-processing pipelines that need guaranteed constant lookups use cuckoo hashing.

Common pitfall

Load factors must stay below roughly 50 percent for two tables, or higher with more choices or buckets, otherwise insertions fail. Insert cost is unpredictable, with occasional full rehashes. Hash functions must be independent. Use stashes or bucketed variants to handle rare failures gracefully.

Where it fits in the course

Cuckoo Hashing is topic 47 of 52 in Data Structures, rated Advanced. It follows Van Emde Boas Trees. The next topic is Count-Min Sketch.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Data Structures quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Cuckoo Hashing?
Hashing scheme with two tables guaranteeing constant lookup. It belongs to Data Structures and is an advanced-level topic.
Where do people use Cuckoo Hashing?
Network hardware lookup tables, high-performance caches, in-memory databases and packet-processing pipelines that need guaranteed constant lookups use cuckoo hashing.
What is the most common mistake with Cuckoo Hashing?
Load factors must stay below roughly 50 percent for two tables, or higher with more choices or buckets, otherwise insertions fail.
What should I learn before this topic?
Start with Van Emde Boas Trees, the topic just before it in Data Structures.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Data Structures quiz. You can also explore Hash Table in the Visual Lab.
Is Cuckoo Hashing hard to learn?
It is an Advanced topic. Expect to read the explanation more than once and to connect it with earlier topics. Working through a small example by hand helps most.
How long does it take to learn Cuckoo Hashing?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Cuckoo Hashing?
Van Emde Boas Trees, Count-Min Sketch, Fibonacci Heaps are closely connected. The full list is in the related topics section of this page.
What should I study after Cuckoo Hashing?
Continue with Count-Min Sketch, the next topic in Data Structures.

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